BOSTON -- Paul Pierce sank the big basket in the Celtics double-overtime win over the NBA champions. Then he learned that star point guard Rajon Rondo would miss the rest of the season with a knee injury.

"Everyone was really happy for the win," Pierce said after Boston beat the Miami Heat 100-98 Sunday. "It brought a dark cloud in this room when you heard the news."

When coach Doc Rivers told his players after the game, the joy of ending their six-game losing streak stopped, even if they had just outlasted the team with the best record in the Eastern Conference.

Now the Celtics must try to keep winning without their leader, who suffered a torn anterior cruciate ligament in his right knee late in Friday night's 123-112 loss, also in double overtime, in Atlanta.

They won Sunday after Pierce's 22-foot jumper with 31 seconds left gave them a 99-98 lead.

But making the playoffs got harder as the Celtics try to hang on to the eighth and final postseason spot in the conference with a 21 2-game lead over the Philadelphia 76ers.

"Obviously, the Rondo news is pretty tough. I knew it before the game," Rivers said. "I just didn't think it was any time to tell any of our guys."

This game was the first in Boston for Ray Allen since he left the Celtics after five seasons and signed as a free agent with Miami. He scored 21 points.

Rondo's injury "puts this team and the rest of the guys in a position to be ready to step up," Pierce said. Sunday's win "was a perfect example. We showed we are capable."

LeBron James had 34 points for the Heat, whose winning streak stopped at four.

"As much as I've been a rival with Boston over the years, I never want to see anyone go down," James said. "It's terrible, not only for their team but for the league."

After Pierce's basket, James had a chance to put the Heat ahead but missed a 12-foot jumper with 6.8 seconds to go from the left with defender Jeff Green jumping out at him. Pierce got the rebound and was fouled by Shane Battier.

He sank the first shot. Then, as a fan shouted "This one's for Rondo," he missed the second.

Miami had one last chance, but Battier missed a long jumper at the buzzer.

"They defended that very well," Heat coach Erik Spoelstra said. "There are about three different options to it, four different options to it. They defended each one of them."

The Heat could have won in the first overtime, but Dwyane Wade, who had 17 points, missed a long jumper as the buzzer sounded. They had led 93-89 after consecutive baskets by James, but Garnett hit a layup with 1:45 remaining and a shot from the right baseline with 1:14 to go.

Boston could have avoided the first overtime when Pierce inbounded from behind his backboard with 2 seconds left to Jason Terry. But Terry's shot from the top of the key was short. The Heat had tied it on a 3-pointer by James with 7 seconds remaining in regulation after Allen missed a 3-pointer from the left corner with 15 seconds to go.

Rivers did not mention Rondo's injury in his meeting with reporters about an hour before the game, but Courtney Lee started in his place. Doctors decided to keep Rondo out of the game after he went through his normal pregame routine but complained of pain that he thought was in his hamstring. An MRI was done and team physician Dr. Brian McKeon learned the results during the game.

"New guys are going to get an opportunity now," Boston forward Paul Pierce said. "These guys haven't had a chance to really showcase what they can do."

For the past five seasons, Pierce, Allen and Kevin Garnett made up the Big Three. After Allen signed with the Heat as a free agent in the offseason, Rondo joined that group -- and became the team leader with his ability to run the offense.

He was chosen as the starting point guard for the Eastern Conference in the All-Star game in Houston on Feb. 17.

Now the Celtics hope he can be ready for the start of next season.

"How far is that?" coach Doc Rivers said, "I don't even know how long that is. Eight months? Nine months? So he'll probably be ready, but I don't know."

Rondo reported to the TD Garden on Sunday for a pregame shootaround but was taken to a hospital after complaining of pain.

The initial report was a hyperextended knee, but tests showed the ACL tear.

Rivers said he learned about 25 minutes before the game that Rondo wouldn't play. Word of the injury's severity came during the game.

"He was telling me he thought he pulled his hamstring," Rivers said, "Rondo will be fine. He just will not be fine this year."

The team made the announcement during the game that snapped a losing streak that had reached six games despite back-to-back triple-doubles by Rondo.

As Celtics players walked off the court through a tunnel toward their locker room, Rondo, dressed in street clothes, greeted them.

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